1. Field of the Invention
The present invention relates to the field of sensors for measuring moisture in an environment and in particular to a method and means for sensing changes in moisture content in an environment utilizing an optical fiber that is sensitive to changes in moisture content.
2. The Prior Art
Sensors are the key component in a wide range of systems used in chemical process control, pollution monitoring, robotics, and chemical warfare agent detection, for example. In response to the development of the need for such systems, advances in sensor technology, and in particular sensors having higher sensitivity and response speed along with reduced size and cost are continually being sought. It has been shown that plasma polymerized thin films deposited from a mixture of hexamethyldisiloxane and ammonia may be useful as sensor membranes for detecting oxidizing gases and moisture. Sensors based on such thin films have exploited the fact that the sheet resistance of the plasma polymerized film decreases gradually with an increase in relative humidity and the fact that such moisture effect is reversible. In one such thin film moisture sensitive detector, having a conductivity read out, the sheet resistance changed from 10.sup.18 to 10.sup.10 .OMEGA. as the relative humidity changed from 0 to 92%. And in such instrument it was shown that the transition between stable readings is rapid.
However, there are a number of applications where it would be desirable to measure moisture, but where conductivity measurement is not practical. An example of such application is in the control of moisture in power transformers with paper-oil insulation. Depending on the operating and ambient conditions, the moisture level in the transformer oil can vary significantly. At present, moisture is determined on batch samples taken from the operating transformer. On-line monitoring of moisture in oil would permit control of moisture build up within desired limits. Another attractive application for a fiber optical moisture sensor is in a hydrogen cooled generator. An on-line moisture sensor would permit signalling of the malfunction or exhaustion of the gas dryer and/or in leakage of ambient air.
Optical fiber sensors have been investigate extensively. And the properties and applications of chemically selective fiber-optic sensors (Optrodes) are presented in an article authored by S.M. Angel and entitled "Optrodes: Chemically Selective Fiber-Optic Sensors" which appeared in Spectroscopy, Vol. 2, No. 4, pp. 38-48, April 1987. Among other advantages, optical fiber sensors have been found to be generally insensitive to electrical interference. Fiber optic chemical sensors have been described using a number of optical phenomena, including absorption across a gap between two fiber ends, fluorescence initiated by light transmitted through an optical fiber and collected by the same fiber, absorbance of light by a substrate at the end of a fiber with observation of the transmitted or reflected light, and a number of other effects. Of particular interest for the present disclosure is the technique of evanescent wave attenuation at the surface of a fiber, whereby the light travelling through the fiber by multiple internal reflections is modulated by the absorption spectrum of the surrounding medium. Sensors exploiting this technique have been employed for detecting color changes in an indicating dye immobilized in a plastic coating on a fiber-optic core in response to pH changes in a solution in which the fiber-optic probe is immersed.
In the past, however, there has been no reported use of optical fibers for sensing moisture.